Food grade lubricating greases are used on machinery or equipment in the food manufacturing and processing industries. It is desirable that a food grade grease have high extreme pressure characteristics, meaning that the grease film located between the operating components of the machinery is capable of withstanding high pressures before rupturing. A food grade grease should also have good xe2x80x9canti-washoutxe2x80x9d properties, meaning that the grease will not readily wash away with the sanitizing solution which is used to wash down the equipment after each day""s run. In addition, it is desirable that a food grade grease be environmentally friendly. In the past, the conventional food grade grease has not met this criteria for the usual food grade grease has relatively low extreme pressure characteristics and does not have high xe2x80x9canti-washoutxe2x80x9d properties, thus requiring that the equipment be frequently re-lubricated.
It is known to add bismuth compounds such as bismuth carboxylates to lubricating oils. For example, U.S. Pat. No. 5,385,683 describes a lubricating oil composition containing a bismuth carboxylate, such as bismuth neodecanoate or bismuth 2-ethylhexanoate, along with a tin carboxylate. The patent states that at high temperatures the bismuth and tin compounds dissociate to form a bismuth-tin alloy that coats the working part.
U.S. Pat. No. 5,576,273 discloses a lubricating oil composition containing a bismuth carboxylate along with a dihydrocarbyidithiocarbamate. This combination is stated to produce improved extreme pressure properties for the lubricating oil. It is also known to incorporate a bismuth carboxylate and a carboxylic acid in lubricating oil compositions to improve the physical properties of the oil.
The invention is directed to a lubricating grease composition and in particular to a food grade grease composition.
In general, the grease composition includes an oil base, a thickener, and bismuth, preferably in the form of a bismuth carboxylate.
The thickener is preferably bentonite clay, although other thickeners, such as lithium soap, an aluminum or calcium complex or polyurea can also be employed. When dealing with a food grade grease, the oil base can take the form of a vegetable oil such as soy bean oil, canola, palm oil, or the like.
In addition, the grease composition can also include small amounts of anti-oxidants or preservative, corrosion inhibitors, fillers, whiteners and/or tackifiers.
At elevated temperatures during service, the bismuth carboxylate will dissociate resulting in the plating of bismuth on the working part. The grease has excellent film strength, providing a rating of about 55 on the Timken rating scale. This compares with a Timken rating generally in the range of about 25 for a typical food grade grease. Due to the unique extreme pressure characteristics of the grease of the invention, the film strength, which is the pressure required to break the lubricating film, is very high, allowing for less metal-to-metal contact of bearing surfaces. Thus, the use of the grease substantially increases the service life of equipment and results in less down time.
The grease also has excellent xe2x80x9canti-washoutxe2x80x9d capabilities. Due to the improved xe2x80x9canti-washout propertiesxe2x80x9d, the frequency of re-lubrication is decreased.
The food grade grease incorporating a vegetable oil such as soy bean oil is also environmentally friendly and thus minimizes pollution of the environment when a portion of the grease may be flushed away with the wash water.
Other objects and advantages will appear during the course of the following description.